Field of the Invention and Related Art Statement
The present invention relates to a magneto-optic player for reading an information signal out of a magneto-optic record medium.
There have been proposed various kinds of magneto-optic players. Some examples are disclosed in Japanese Patent Publications Kokai-sho 63-184936 and 63-205836, and U.S. Pat. Nos. 3,284,785, 3,525,870 and 4,774,615.
FIG. 1 shows the construction of a known magneto-optic player. In this known magneto-optic player, laser light emitted from a semiconductor laser 1 is projected upon a magneto-optic player 5 by means of a collimator lens 2, a beam splitter 3 and an objective lens 4. Light reflected by the magneto-optic record medium 5 is made incident upon a polarization beam splitter 7 by means of the objective lens 4, the beam splitter 3 and a halfwave plate 6. Light beams transmitted through and reflected by the polarization beam splitter 7 are received by photodetectors 8aand 8b, respectively. Output signals from the photodetectors 8a and 8b are supplied to a differential amplifier 9 to derive a reproduced signal. In this manner, it is possible to derive a reproduced information signal in the differential manner. In this known magneto-optic player, the halfwave plate 6 is arranged such that an orientation of an optical axis of a crystal of the plate is set to 22.5 degrees with respect to the polarization direction of the linearly polarized incident laser light impinging upon the magneto-optic record medium 5, so that the polarization plane of the laser light impinging upon the polarization beam splitter 7 is rotated by 45 degrees with respect to the incident laser light.
FIG. 2A to 2G illustrate the polarized conditions and Jones vectors of polarized light beams traveling along various light paths in the known magneto-optic player shown in FIG. 1. It is assumed that the linearly polarized laser light emitted by the semiconductor laser 1 has a unit amplitude of 1 as shown in FIG. 2A. This laser light is transmitted through the beam splitter 3 as the linearly polarized light as illustrated in FIG. 2B. The amplitude of the transmitted laser light can be represented by .sqroot.T.sub.P, wherein T.sub.P is the transmissivity of the beam splitter 3. The laser light reflected by the magneto-optic record medium 5 is shown in FIG. 2C, wherein R represents the reflectance of the record medium 5 and .+-..theta..sub.K denote the Kerr rotation angles, the positive and negative signs being determined in accordance with the direction of the vertical magnetization in the record medium. That is to say, when the information signal is recorded in one direction of the vertical magnetization, the polarization plane of the incident linearly polarized light is rotated by +.theta..sub.K, while when the signal is recorded in the opposite direction of magnetization, the incident light is subjected to the Kerr rotation of -.theta..sub.K. When the laser beam is reflected by the beam splitter 3, the laser light represented in FIG. 2D can be obtained, in which R.sub.P and R.sub.S denote the reflectances of the beam splitter 3 for P-polarized light and S-polarized light, respectively. It should be noted that the reflectance for the S-polarized light is assumed to be 100%. The polarization plane of the light reflected by the beam splitter 3 is rotated by 45 degrees by means of the halfwave plate 6 as shown in FIG. 2E. The light transmitted through the halfwave plate 6 is made incident upon the polarization beam splitter 7 and is separated into the P-polarized light (transmitted light) and S-polarized light (reflected light) as illustrated in FIGS. 2F and 2G. By detecting these light beams with the aid of the two photodetectors 8a and 8b and then deriving the difference between the outputs of the photodetectors, it is possible to reproduce the information signal having a rather high C/N ratio in the differential manner.
However, in the known magneto-optic player shown in FIG. 1, it has been confirmed that the C/N ratio of the reproduced information signal is greatly affected by a deviation of the orientation of the halfwave plate 6, so that the halfwave plate has to be adjusted very precisely. This will be further explained in detail.
As explained above, the halfwave plate 6 should be arranged such that the orientation of the optical axis of the crystal plate is set to 22.5 degrees with respect to the linear polarizing direction of the laser light emitted by the semiconductor laser 1. If the setting angle of the halfwave plate 6 is deviated by an angle of .alpha. degrees and the optical axis of the crystal plate is changed to 22.5+.alpha., the rotation angle of the polarization plane of the incident light due to the Kerr rotation becomes 45+2.alpha. degrees. Then, the amplitude of the reproduced signal obtained by deriving the difference between the output signals from the photodetectors 8a and 8b can be expressed as follows. ##EQU1## Further, an amount of differential unbalance of intensity of light beams impinging upon the two photodetectors 8aand 8b can be represented by the following equation. EQU RT.sub.P sin 4.alpha.(R.sub.P cos.sup.2 .theta..sub.K -R.sub.S sin.sup.2 .theta..sub.K) (2)
As can be understood from the above mentioned two equations (1) and (2), when the halfwave plate 6 has a setting error of .alpha., the intensity of the reproduced signal is decreased in proportion to cos 4.alpha. and the differential unbalance amount is increased in proportion to sin 4.alpha.. Therefore, even if the deviation angle .alpha. is very small, there is produced a large amount of noise and the C/N ratio of the reproduced signal is decreased to a great extent.
As explained above, in the known magneto-optic player shown in FIG. 1, the C/N ratio of the reproduced signal is decreased to a great extent due to the deviation of the orientation of the halfwave plate 6, so that the arrangement of the halfwave plate has to be adjusted very precisely and thus the assembling of the player becomes very cumbersome and the cost of the player is liable to be high.
In order to avoid the above explained drawback, one possibility is to delete the halfwave plate 6 and the polarization beam splitter 7 is rotated by 45 degrees about the optical axis in the drawing of FIG. 1. In such arrangement, the influence of the adjustment error of the optical system is reduced to half of that of the player shown in FIG. 1, and therefore the precision of arrangement can be reduced by a factor of two. However, in such an arrangement, the light reflected by the polarization beam splitter 7 is bent by 45 degrees with respect to the plane of the drawing of FIG. 1, and thus the optical elements could not be arranged in the same plane, so that the overall construction of the optical system becomes large.